We are engaged in a study of paramyosin-containig invertebrate muscles. Our goal is to elucidate this protein's role in: 1) determining thick filament structure; 2) allowing or bringing about changes in thick filament dimensions which occur under physiologic conditions in some arthropodan muscles; and 3) modulating thick-thin filament interactions. We have established the presence of paramyosin in a variety of arthropodan and molluscan muscles, demonstrated that the molecule's immunological and dimensional properties are highly conserved in these muscles, localized the paramyosin to the A-bands and determined the paramyosin/myosin molecular ratios for all of these muscles, showing a correlation between this ratio and thick filament length. Our current projects, most of them projected to continue into the coming year, include: 1) a continuing survey of paramyosin in muscles of other phyla; 2) development of an immunoelectrophoretic procedure to determine, routinely and rapidly, the identity of bands in SDS-polyacrylamide gels; 3) completion of our analysis of two lobster abdominal muscles which differ from each other in their anatomy, physiology and paramyosin content; 4) thick filament reconstitution, using varying ratios of myosin and paramyosin; and 5) determination of paramyosin's effect on acto-myosin ATPase activity. Completion of these projects will aid us in understanding the role of paramyosin.